Notes on SQL

Entity relationship diagrams

The “crow’s foot” that connects the tables together shows us how the columns in one table relate to the columns in another table. ## Types Of Statements A few statements include: * CREATE TABLE is a statement that creates a new table in a database. * DROP TABLE is a statement that removes a table in a database. * SELECT allows you to read data and display it. This is called a query.

SELECT & FROM Statements

Here you were introduced to two statements that will be used in every query you write * SELECT - what columns you want back. * FROM - what table you are querying from.

SELECT id, account_id, occurred_at
FROM orders;

LIMIT Statement

The LIMIT statement is useful when you want to see just the first few rows of a table. This can be much faster for loading than if we load the entire dataset. The LIMIT command is always the very last part of a query. An example of showing just the first 10 rows of the orders table with all of the columns might look like the following:

SELECT *
FROM orders
LIMIT 10

ORDER BY Statement

• The ORDER BY statement allows us to order our table by any row.
• Remember DESC can be added after the column in your ORDER BY statement to sort in descending order, as the default is to sort in ascending order.

SELECT id, occurred_at, total_amt_usd
FROM orders
ORDER BY occurred_at
LIMIT 10;

We can ORDER BY more than one column at a time. The statement sorts according to columns listed from left first and those listed on the right after that.

SELECT *
FROM orders
ORDER BY occurred_at, total_amt_usd
LIMIT 10;

WHERE Statements

Using the WHERE statement, we can subset out tables based on conditions that must be met.

SELECT *
FROM orders
WHERE gloss_amt_usd >= 1000
LIMIT 5;

The WHERE statement can also be used with non-numerical data. We can use the = and != operators here. You also need to be sure to use single quotes

SELECT name, website, primary_poc
FROM accounts
WHERE name = 'Exxon Mobil';

Arithmetic Operators

Derived Columns

Creating a new column that is a combination of existing columns is known as a derived column. ### Order of Operations Remember BOMDAS from maths class? The same order of operations apply when using arithmetic operators in SQL.

SELECT id, account_id, 
       poster_amt_usd/(standard_amt_usd + gloss_amt_usd + poster_amt_usd) AS post_per
FROM orders;

Introduction to Logical Operators

LIKE - This allows you to perform operations similar to using WHERE and =, but for cases when you might not know exactly what you are looking for. The LIKE operator is frequently used with %. The % tells us that we might want any number of characters leading up to a particular set of characters or following a certain set of characters.

/* select all the companies whose names start with 'C' */
SELECT name 
FROM accounts
WHERE name like 'C%'

/* select All companies whose names contain the string 'one' somewhere in the name. */
SELECT name
FROM accounts
WHERE name LIKE '%one%';

IN - This allows you to perform operations similar to using WHERE and =, but for more than one condition. We can check one, two or many column values for which we want to pull data, but all within the same query. TIP - In most SQL environments, you can use single or double quotation marks - and you may NEED to use double quotation marks if you have an apostrophe within the text you are attempting to pull.

SELECT name, primary_poc, sales_rep_id
FROM accounts
WHERE name IN ('Walmart', 'Target', 'Nordstrom');

NOT - This is used with IN and LIKE to select all of the rows NOT LIKE or NOT IN a certain condition.

/* Use the accounts table to find the account name, primary poc, and sales rep id for all stores except Walmart, Target, and Nordstrom */
SELECT name, primary_poc, sales_rep_id
FROM accounts
WHERE name NOT IN ('Walmart', 'Target', 'Nordstrom');

AND & BETWEEN - These allow you to combine operations where all combined conditions must be true. * The AND operator is used within a WHERE statement to consider more than one logical clause at a time. Each time you link a new statement with an AND, you will need to specify the column you are interested in looking at. * Sometimes we can make a cleaner statement using BETWEEN than we can using AND. Particularly this is true when we are using the same column for different parts of our AND statement. e.g.:

/*Instead of writing : WHERE column >= 6 AND column <= 10 we can instead write*/ 
WHERE column BETWEEN 6 AND 10

/* Write a query that returns all the orders where the standard_qty is over 1000, the poster_qty is 0, and the gloss_qty is 0 */
SELECT *
FROM orders
WHERE standard_qty > 1000 AND poster_qty = 0 AND gloss_qty = 0;

/* Using the accounts table find all the companies whose names do not start with 'C' and end with 's' */
SELECT name
FROM accounts
WHERE name NOT LIKE 'C%' AND name LIKE '%s';

/* Use the web_events table to find all information regarding individuals who were contacted via organic or adwords and started their account at any point in 2016 sorted from newest to oldest */ 
SELECT *
FROM web_events
WHERE channel IN ('organic', 'adwords') AND occurred_at BETWEEN '2016-01-01' AND '2017-01-01'
ORDER BY occurred_at DESC;
/*  You will notice that using BETWEEN is tricky for dates! While BETWEEN is generally inclusive of endpoints, it assumes the time is at 00:00:00 (i.e. midnight) for dates. This is the reason why we set the right-side endpoint of the period at '2017-01-01'. */

OR - This allow you to combine operations where at least one of the combined conditions must be true. Similar to the AND operator, the OR operator can combine multiple statements. Each time you link a new statement with an OR, you will need to specify the column you are interested in looking at.

/* Find list of orders ids where either gloss_qty or poster_qty is greater than 4000. Only include the id field in the resulting table. */
SELECT id
FROM orders
WHERE gloss_qty > 4000 OR poster_qty > 4000;

/* Write a query that returns a list of orders where the standard_qty is zero and either the gloss_qty or poster_qty is over 1000. */
SELECT *
FROM orders
WHERE standard_qty = 0 AND (gloss_qty > 1000 OR poster_qty > 1000);

/* Find all the company names that start with a 'C' or 'W', and the primary contact contains 'ana' or 'Ana', but it doesn't contain 'eana'*/
SELECT *
FROM accounts
WHERE (name LIKE 'C%' OR name LIKE 'W%') 
           AND ((primary_poc LIKE '%ana%' OR primary_poc LIKE '%Ana%') 
           AND primary_poc NOT LIKE '%eana%');

Summary of basic comands:

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JOINS

Inner joins

Only return rows that appear in both tables e.g. Using ERD: Pull standard_qty, gloss_qty, and poster_qty from the orders table, and the website and the primary_poc from the accounts table.

SELECT orders.standard_qty, orders.gloss_qty, 
       orders.poster_qty,  accounts.website, 
       accounts.primary_poc
FROM orders
JOIN accounts
ON orders.account_id = accounts.id

Notice that we need to specify every table a column comes from in the SELECT statement. ### ALIAS When we JOIN tables together, it is nice to give each table an alias. Frequently an alias is just the first letter of the table name.

FROM tablename t1 
JOIN tablename2 t2

Questions

1. Provide a table for all web_events associated with account name of Walmart. There should be three columns. Be sure to include the primary_poc, time of the event, and the channel for each event. Additionally, you might choose to add a fourth column to assure only Walmart events were chosen.

SELECT a.primary_poc, w.occurred_at, w.channel, a.name
FROM web_events w
JOIN accounts a
ON w.account_id = a.id
WHERE a.name = 'Walmart';

2. Provide a table that provides the region for each sales_rep along with their associated accounts. Your final table should include three columns: the region name, the sales rep name, and the account name. Sort the accounts alphabetically (A-Z) according to account name.

SELECT r.name region, s.name rep, a.name account
FROM sales_reps s
JOIN region r
ON s.region_id = r.id
JOIN accounts a
ON a.sales_rep_id = s.id
ORDER BY a.name;

3. Provide the name for each region for every order, as well as the account name and the unit price they paid (total_amt_usd/total) for the order. Your final table should have 3 columns: region name, account name, and unit price. A few accounts have 0 for total, so I divided by (total + 0.01) to assure not dividing by zero.

SELECT r.name region, a.name account, 
       o.total_amt_usd/(o.total + 0.01) unit_price
FROM region r
JOIN sales_reps s
ON s.region_id = r.id
JOIN accounts a
ON a.sales_rep_id = s.id
JOIN orders o
ON o.account_id = a.id;

Outer joins

If we want to include data that only exists in one of the tables we ant to join we can use an outer join If there is not matching information in the JOINed table, then you will have columns with empty cells with a data type called NULL. Writing a LEFT join statement: Comparing LEFT and RIGHT join: